In the latter half of 1970 years, research and development of manufacturing gas sensors by using Si-semiconductor processes in the rising period were taken place vigorously but they were declined except for ISFET used in live blood analysis. This was attributable mainly to that since the gas sensors were operated at high temperature (about 450° C.) in most cases crack or defoliation occurred in the thin film and long-term stability could not be achieved. The problem of peeling and cracking of the thin films was caused since the gas sensors had small heat capacity and poor thermal shock for the sake of the thin film, as well as that the process mainly includes dissolving fine grains in a solvent, and coating and sintering them in which no sufficient adhesive property between the fine grains and the substrate or the electrode can be ensured (for example, refer to Yasuhiro Shimizu, Makoto Egashira, Applied Physics, Vol. 70, No. 4, pp. 423 to 427, 2001 (Non-Patent Literature 1)). However, the technique of micro electro mechanical systems (MEMS) has been started for application to the development of the gas sensors about in the latter half of 1990 years and research and development therefor have become active again as a tramp for the low consumption power (for example, refer to I. Simon, et al., Sensors and Actuators B Vol. 73, pp. 1 to 26, 2001 (Non-Patent Literature 2), and T. Suzuki, et al. “10th Int. Meeting Chemical Sensors, 3B02, Jul. 11 to 14, 2004, Tsukuba, Japan” (Non-Patent Literature 3).
Most of current gas sensors adopt a catalytic combustion type, a metal-oxide semiconductor type, a gas thermal conduction type, and a solid electrolyte type. However, if various types of gas sensors can be achieved based on the Si-MOSFET technique (silicon semiconductor and integrated technique thereof), since a sensor sensitive portion can be formed of a hyper thin film and Si lithography can be utilized, it may be excepted that this can bring about a revolution in the field of the gas sensors while taking advantageous features of the Si semiconductor technique such as possibility of providing micro miniaturization, weight reduction, low consumption power, codeless operation (battery operation), portability, network adaptability, and mass production at low cost.
Actually, Si-MOSFET gas sensors for detecting NH3, CO, CH4, and NO gases by making the Pt gate thin film into a porous structure have been proposed though at a laboratory scale (Non-Patent Literatures 4, 5, 6). The principle of the sensor is that Pt small crystals are formed with an air gap therebetween over a gate insulating film and, when a gas molecule is adsorbed on Pt, a work function changes to shift the Vth of MOSs through the capacitance between the Pt small crystals.
An Si-MOSFET hydrogen gas sensor using a platinum film comprising a thin film at a thickness of about 30 to 45 nm for a gate electrode does not respond to ammonia, ethane, methanol, etc. except for a hydrogen gas. However, gas selectivity is different in the Si-MOSFET hydrogen gas sensor using a hyper thin platinum film (down to 6 nm) for a gate electrode (for example, refer to Sensors and Actuators B, Vol, pp. 15 to 20, 1990 (Non-Patent Literature 7). That is, according to their study, since the platinum thin film is not deposited uniformly over the gate insulating film and formed in a stripe shape, an air gap region with no platinum is present on the surface of the gate insulating film, and gas sensors capable of detecting also ammonia, ethane, methanol, etc. other than the hydrogen gas have been manufactured by utilizing the structure. It can be said that this is a gas sensor of positively utilizing the nature that the platinum film has poor adhesive property and tends to cause film peeling on the gate insulating film. The response mechanism of a gas sensor is as shown below as discussed in the literature (Non-Patent Literature 7) and other reference documents.
That is, ammonia gas or the like is deposited to the platinum surface formed in a stripe shape to change the surface potential φs on the platinum surface. In this case, the threshold value Vth changes, in principle, depending on static capacitance between the gap region with no platinum and the platinum small crystal at the surface of the gate insulating film and a static capacitance between the gap region with no platinum and the channel formed in the semiconductor substrate (Si substrate). However, also in the gas sensor using the hyper thin film (down to 6 nm) of the platinum, the problem of the reliability such as peeling of the platinum film has not yet been solved and the sensor still involves a problem and it is difficult to be put to practical use.
The Si-MOSFET hydrogen gas sensor using platinum as the gate electrode cannot be put to commercial product, because the adhesion property between the insulating film comprising, for example, silicon oxide and a semiconductor film comprising, for example, silicon or gallium arsenide (GaAs), etc. is poor and long-term reliability cannot be ensured. The problem of peeling of the platinum film is an extremely important problem also in view of the practical use and the ensurance of the working life in the gas sensor in which the gate electrode portion is exposed to an atmospheric air.
Further, partial film peeling is caused in the fabrication process of the FET manufacturing steps, and a technique of stably bonding Pt directly on the gate insulating film has not yet been established in practical production steps. Peeling of Pt film results in a problem of contamination to process apparatus due to the Pt film exfoliated in the manufacturing steps also in view of the manufacturing method, and a technique of avoiding the contamination had been established by inserting a barrier metal such as Ti, Mo, and W between Pt and an oxide such as SiO2 or a semiconductor such as Si or GaAs thereby maintaining the adhesion in a case of using Pt in the field of electronic devices using Si, GaAs or the like. Pt is a noble metal and tends to become more stable when Pt is agglomerated per se than in a state of bonding with oxygen or other constituent atoms in solid materials (oxide such as SiO2 and semiconductor such as Si and GaAs). This is a nature inherent to Pt and insertion of the barrier metal for improving the adhesion property is an essential method.
In view of the operation of the hydrogen sensor, there is a primary problem that a hydrogen gas is blocked or occluded in the barrier metal layer if the barrier metal layer is present and the sensor does not respond to hydrogen gas at all or the hydrogen response sensitivity is extremely lowered to inhibit the use as the sensor.
On the other hand, we also performed air annealing to a Pt (15 nm)/Ti (5 nm)/SiO2 (18 nm)/Si stacked film MOS structure at 800° C. for 30 minutes to achieve a porous structure (for example, in Patent Literature 1, FIG. 18 shows a sensor principle explanatory view and FIG. 19 shows a cross sectional TEM image of gate). We have also performed air annealing to a Pt (15 nm)/Ti (5 nm)/SiO2 (18 nm)/Si stacked MOS structure at 400° C. for 2 hours to achieve a hydrogen sensor using a Pt—Ti—O gate structure in which a Ti layer comprises a mixed layer of TiOx nanocrystals and an amorphous Ti doped with oxygen at an ultrahigh concentration, in which Ti and O are accumulated at high concentration in the Pt grain boundary to achieve extremely high sensitivity characteristics at 100 ppm to 1% hydrogen concentration diluted with air (for example, in Patent Literature 1, FIG. 12 shows a concentration dependency and high sensitive characteristics of sensor and FIG. 1 explain the gate structure thereof and FIG. 2 shows a cross sectional TEM image of the gate). This Pt—Ti—O gate structure has excellent characteristics having a life time of the intrinsic chip for 10 years or more as the characteristics thereof (for example, refer to Usagawa, et al. Fuel Cell, Vol 8, No. 3, 2009, pp. 88 to 96 (Non-Patent Literature 8)), and it was found that the reproducibility of the threshold voltage Vth and the uniformity in wafer were improved outstandingly by applying a hydrogen annealing treatment (for example, in Patent Literature 2, FIG. 7(a)). However, the Pt—Ti—O gate structure does not respond to 0.1 to 1.0% methane, 0.1% ethane, 0.1% CO, and 817 ppm isooctane at an operation temperature of 115° C. (Non-Patent Literature 8).
The problems of the barrier metal have been solved by using the Pt—Ti—O gate structure invented by the technique in the prior application (Patent Literature 1).